Of AC Motors and Capacitors…

Some interesting lessons learned about start capacitors, run capacitors, and AC motors.  My pain is your gain… things are not always what they appear, and cheaper is not necessarily better!

The Patient

In my workshop, I’ve got a Delta Shopmaster AP400 dust collector that recently failed to start.  It just sits there and hums.

The basic checks turned up no obstructions, and the impeller turned freely, though I wasn’t inclined to spin-start it by hand.  Initial diagnosis: failed “start” capacitor.  (Later, a safer test confirmed that the motor would run in either direction if started by hand.)

First Attempt

Not knowing a lot about AC motors, I did some digging and confirmed that the AP400 does tend to have this failure.  Not surprising; it’s a common failure of AC motors.  Everybody seems to be using the same term, “start capacitor”.  Without a start capacitor, an AC motor doesn’t know which direction to start turning, and it just sits there.  I must be on the right trail!

Well, the part shops wanted $25 for a factory replacement.  Being industrious (and impatient, and cheap), I found an “identically” spec’d start capacitor at my local Grainger for $8.  I installed it and went on my way, happy at my cheap fix…  It ran for about 5 minutes until the stench of blown capacitor filled the shop.

I’d learned that start capacitors were supposed to disengage after the motor was up to speed.  Clearly, this one wasn’t disengaging, and it was being run to death immediately.  New diagnosis: the centrifugal switch inside the motor must be stuck, and the start capacitor was blowing as a result.  Another trip to Grainger for a new capacitor, anticipating that I’d fix the internal switch over the weekend.

Round Two

New capacitor in-hand, I tore into the motor itself.  A handful of screws and some love taps with a mallet got it apart.  And… and… no centrifugal switch!  The capacitor is wired directly into the secondary windings.  It couldn’t be a simpler design.

I began to worry that the secondary winding might have a short in it, and I’d be replacing the motor soon (for more than I paid for the entire unit).  I started to hatch a plan to install a pushbutton to manually engage the capacitor only during startup-up.

A Discovery

More research revealed a surprise – the pundits were using the wrong term.  This was not a “start capacitor” and the motor isn’t a capacitor-start design.  It’s a Permanent-Split Capacitor (PSC) design, and the capacitor is a “run capacitor” – it’s designed to be in the circuit full-time.

Why does this matter?  After all, the new capacitor had identical specs.  Or did it?  Apparently not.

It turns out, capacitors marketed as “start capacitors” are rated for a duty cycle – number of starts per hour.  “Run capacitors” aren’t rated like this because they’re designed to be energized full-time.  They’re also physically larger.  So, it turns out that I’d bought the wrong kind of capacitor, and that’s why the new one got cooked.

The Solution

I sucked it up and paid the $25 for a factory replacement.  At this point, the project was a failed attempt at frugality, though it was a great learning experience.  The proper run capacitors from Grainger turned out to be physically too large for the motor controller enclosure, and their price was nearly the same.

Some tips learned

  • When spec’ing a replacement capacitor, be sure it’s the right type – Run vs. Start.  When replacing a capacitor, the old one won’t be marked as “start” or “run”, so some research is required for the motor being serviced.  Unfortunately, my initial research turned up bad info.
  • Caps have two main ratings – capacity and max voltage.  You need to match the capacity within 10%, but it’s OK to exceed the voltage rating.  (The AP400’s motor uses one rated 50µF at 250v.)
  • For safety, the voltage rating should be 50% higher than the actual normal voltage, to handle spikes. (e.g., 250v for 120v use, 370v for 240v use, etc.)  Since the AP400 can be wired for 240v, it seems odd that the factory run capacitor isn’t rated for higher voltage, but I don’t care since I’m running it at 120v.
  • Capacity is measured in microfarads, properly marked as µF but also commonly marked as MFD.  More is not better here – an over-sized capacitor will be mis-matched to the motor’s design and may work against it; it will also dump more power into the winding than designed, causing it to heat more and fail faster.  There is no magic formula for determining the best capacity – it’s recommended by the manufacturer based on the specific motor and its application.
  • Start capacitors are typically rated a lot larger than run capacitors – like 250µF instead of 50µF, or 50µF instead of 5µF (like in an air conditioning blower, which often has both types).

Hopefully this helps someone else buy the right part and keep their dust collector alive for another several years.  If so, drop me a comment here and let me know!


19 Responses to “Of AC Motors and Capacitors…”

  1. Bill Davis Says:

    Thanks. Good explanation an helpful summary.

  2. George Says:

    Thanks for the tip, I’ll be trying this fix out myself – the start capacitor has a bulge in the white plastic housing – you may have saved me the 2nd fix by ensuring I get the correct capacitor the first time!

  3. This looks to be a common problem with the AP400 and other Delta machines using a 1HP (and lower) AC motor.

    I landed here because my AP400 just toasted it’s original run capacitor… this is an excellent journal/tutorial!

    Lessons learned in my adventures… it’s downright difficult if not impossible to find an exact replacement cap that will easily fit the dimensions of the AP400 switch enclosure. There are no twin wire lead run capacitors being sold , unless you buy the factory replacement you will need to retrofit the wiring to use spade connectors and insulate them from the metal switch housing.

    You may pay more initially, but in the long run, the factory replacement is they way to go.. you’ll have your machine up and running long before you find an EOM replacement and it will be much less stressful and much less of a hassle.

    • Hi, Conrad.
      You’re exactly correct. I tried to save money by sourcing my own capacitor, but in the end I found that a capacitor intended for this model was not as overpriced as I thought. It wasn’t worth my time to try otherwise.
      This seems to be a common failing of AC motors in general, though of all the tools in my shop this is the only one that’s had a failed capacitor. I can’t say whether it’s normal or a design issue, but spending around $25 every 15-20 years to fix it isn’t too bad.
      Cheers, Richard

  4. Excellent diagnosis, must be something about AP400 machines. I wish I read this before taking the machine apart and destroying a run capacitor.

  5. This part is out of stock everywhere I’ve checked. Does anyone have any leads on getting ahold of one?

    Thanks so much for such a great writeup! Although I went through the same process you described before finding this page, it saved me re-wiring and doing it once more.

    • Have you tried ordering it from Delta? I wasn’t able to source one with leads from anywhere else, and their price turned out not to be as excessive as first appeared. It’s the right size/shape too.

      • Delta and everyone else is on backorder, but check this out…

        I got this capacitor, drilled a hole in the control box, covered the terminals with a PVC end cap, and wired it in. SUCCESS!

        Runs as cool as you’d want – ran it for a solid 20 min, long enough for it to have heated up I think, and no issues so far.


      • Hi, Mike.
        The specs on that capacitor look good from what I’ve learned, with proper headroom for running at 240v as well. I think the physical size is what pushed me to finally buy the Delta version – like you found, the alternatives don’t fit in the control box (requiring a mod like you did). I’m sure that if Delta had been out of stock, I’d have done exactly what you did. Cheers, Richard

  6. Silvester Wanjalah Says:

    Very nice! its very helpful, from now iam now able to trouble shoot a capacitor just by eye as being a technician in rac. Thanks.

  7. Hello Richard,

    I have the same dust collector and basically the same story as you. I thought I was a hero until it went ‘pop’ and filled my shop with the same acrid smell (what is that, anyway ?). I just wish I had discovered this post beforehand. Could have saved myself $8 and a trip to Grainger!

    Thanks so much,


    • Hi, Gary. I’m glad the post helped eventually. 🙂 I believe that smell is boiled electrolyte from between the capacitor’s rolled up “plates”, and the “pop” is from the capacitor blowing its safety valve to let the “magic smoke” out. (The top is usually scored, apparently to enable exactly this for exactly these reasons.) Nobody really knows where magic smoke comes from, but it’s a proven fact that if you let it out the part won’t keep working. 😉
      Cheers, Richard

  8. Thanks for the info, much appreciated

  9. Joe Diaz Says:

    I want to thank you for the info. from the first paragraph I read I knew I had found my solution. I too have a Delta AP400 that the motor just quit. It is doing the same exact thing as yours, so I’m sure my capacitor is shot.

    Just one question. Is it possible for you to send me the or post the part number for that capacitor. I’d hate to order and get the wrong one.

    Thanks again for the great informative post.

  10. Steve Says:

    Hi, I’m so glad I came across your site! I have a Delta AP400 Dust collector with the same capacitor issue. And same as some of the other people on here, I replaced my capacitor with a ‘start’ capacitor not realizing I needed a run capacitor. The capacitor that was in there had the following specs:

    Can someone please provide a link to an adequate replacement? I don’t know too much about these things and I want to make sure I purchase the correct run capacitor.


    • Hi, Steve. I see the Delta-branded capacitor that was linked may no longer be available. Going from specs, here’s one on Amazon that should do the job – you’ll need to crimp a female spade connector onto your wire. https://www.amazon.com/QX6550-Capacitor-Motor-Round-Volt/dp/B076TC6Z77/

      In case this link fails in the future, I found it by searfching for “50uF run capacitor” and finding one with the same capacity (50uF) and at least 250v rating. At least two of the other values are temperature range ratings (-25ºC to +70ºC), and just about anything made will be fine for indoor temps.

  11. Here it is 2022 and my AP400 capacitor failed. There seems to be plenty of replacements on Amazon. The problem is the wires are both white and the motor wires are red. No marking as to which wire is which. How did you know which wires to hook up. I sure don’t want to wire it backwards. Thanks.

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